Ziegler-Natta (ZN) type polyolefin catalysts are well known in the field of polymers, generally, they comprise (a) at least a catalyst component formed from a transition metal compound of Group 4 to 6 of the Periodic Table (IUPAC, Nomenclature of Inorganic Chemistry, 1989), a metal compound of Group 1 to 3 of the Periodic Table (IUPAC), and, optionally, a compound of group 13 of the Periodic Table (IUPAC) and/or an internal donor compound. ZN catalysts may also comprise (b) further catalyst component(s), such as a cocatalyst and/or an external donor.
Various methods for preparing ZN catalysts are known in the state of art. In one known method, a supported ZN catalyst system is prepared by impregnating the catalyst components on a particulate support material. In WO-A-01 55 230, the catalyst component(s) are supported on a porous, inorganic or organic particulate carrier material, such as silica.
In a further well known method the carrier material is based on one of the catalyst components, e.g. on a magnesium compound, such as MgCl2. This type of carrier material can also be formed in various ways. EP-A-713 886 of Japan Olefins describes the formation of MgCl2 adduct with an alcohol which is then emulsified and finally the resultant mixture is quenched to cause the solidification of the droplets.
Alternatively, EP-A-856 013 of BP discloses the formation of a solid Mg-based carrier, wherein the Mg-component containing phase is dispersed to a continuous phase and the dispersed Mg-phase is solidified by adding the two-phase mixture to a liquid hydrocarbon.
The formed solid carrier particles are normally treated with a transition metal compound and optionally with other compounds for forming the active catalyst.
Accordingly, in case of external carriers, some examples of which are disclosed above, the morphology of the carrier is one of the defining factors for the morphology of the final catalyst.
One disadvantage encountered with the supported catalyst systems is that a possible surface treatment (impregnation step) of the support with one or more catalytically active compounds may lead to non-uniform distribution of the active component(s) within the catalyst particle and further between the separate particles (intra- and inter-particle inhomogeneity) and in turn to an inhomogeneous polymer material.
Further, support material will remain in the final polymer as a residue, which is harmful in some polymer applications.
WO-A-00 08073 and WO-A-00 08074 describe further methods for producing a solid ZN-catalyst, wherein a solution of an Mg-compound and one or more further catalyst compounds are formed and the reaction product thereof is precipitated out of the solution by heating the system. Furthermore, EP-A-0 926 165 discloses another precipitating method, wherein a mixture of MgCl2 and Mg-alkoxide is precipitated together with a Ti-compound to give a ZN catalyst.
EP-A-0 083 074 and EP-A-0 083 073 of Montedison disclose methods for producing a ZN catalyst or a precursor thereof, wherein an emulsion or dispersion of Mg and/or Ti compound is formed in an inert liquid medium or inert gas phase and said system is reacted with an Al-alkyl compound to precipitate a solid catalyst. According to examples said emulsion is then added to a larger volume of Al-compound in hexane and pre-polymerized to cause the precipitation.
In general, a drawback of such precipitation methods is the difficulty to control the precipitation step and thus the morphology of the precipitating catalyst particles.
Furthermore, the precipitation of the catalyst component(s) results in formation of a broad particle size distribution of catalyst particles comprising particles from very small particles to big agglomerates. Further, the morphology of the catalyst would then of course be lost. In polymerization processes this causes in turn undesired and harmful disturbances, like plugging, particle agglomeration, formation of polymer layers on the walls of the reactor etc., and also in lines and further equipment.
U.S. Pat. No. 5,413,979 describes a further method for the preparation of a solid procatalyst composition wherein support materials are impregnated with catalyst component precursors in order to obtain a catalyst component.
U.S. Pat. No. 4,294,948 finally discloses a process for preparing an olefin polymer or copolymer, employing a solid titanium catalyst component prepared by treating pulverized catalyst precursors with organometallic compounds of a metal of any of groups I or III of the Periodic Table, characterized in that the catalyst preparation occurs using pulverized, solid and particulate precursor materials.
EP-A-1 403 292, EP-A-0 949 280, U.S. Pat. Nos. 4,294,948, 5,413,979 and 5,409,875 as well as EP-A-1 273 595 describe processes for the preparation of olefin polymerization catalyst components or olefin polymerization catalysts as well as processes for preparing olefin polymers or copolymers.
Further several documents describe a special emulsion/solidification technology. WO 03/000757 as well as WO 03/000754 describes a process for the preparation of an olefin polymerization catalyst component, enabling to prepare solid particles of a catalyst component comprising a group 2 metal together with a transition metal, using emulsion/solidification technology.
WO 2004/029112 discloses a further process for preparing an olefin polymerization catalyst component using emulsion/solidification technology, wherein the process is further characterized in that a specific aluminium alkyl compound is brought into contact with the catalyst component, enabling a certain degree of activity increase at higher temperatures.
EP-A-1 862 481 describes also a process for preparing an olefin polymerization catalyst component using emulsion/solidification technology, wherein the control of catalytic activity is achieved by decreasing the amount of titanium present in the solidified particles of the olefin polymerization catalyst component being present in the oxidation state +4 by adding a reducing agent.
EP-A-1 862 482 describes a process for preparing an olefin polymerization catalyst component using emulsion/solidification technology, wherein the process is further characterized in that a specific aluminium alkoxy compound is brought into contact with the catalyst component, enabling a certain degree of activity increase at higher temperatures.
For typical ZN catalyst systems it is known that the control of the molecular weight distribution (MWD), in particular if narrow MWD polymers are desired, is difficult to accomplish, so that for typical narrow MWD polyolefin materials single site catalysts (SSC) are used. Since these single site catalysts are much more expensive, more poison sensitive and more difficult to operate in the plant than ZN catalysts it would however be a great advantage if also ZN catalysts, which allow a control of MWD, in particular in combination with reasonable high catalyst activity, would be available.
In EP-A-1 717 269 it is described that with catalysts, prepared according to WO 03/000754, WO 03/000757 and especially WO 2004/029112, it is possible to obtain polymer compositions, preferably propylene homo- or copolymers, more preferably propylene homopolymers, with a narrower MWD distribution compared to polymer compositions obtained with other Ziegler-Natta catalysts. Furthermore the polymer compositions have a decreased xylene soluble (XS) content compared to polymer compositions obtained with other Ziegler-Natta catalysts.
According to the Examples of EP-A-1 717 269 MWDs between 4.0 and 7.1 have been achieved using catalysts according to WO 03/000754, WO 03/000757 and WO 2004/029112 for the polymerization of propylene in combination with triethyl aluminium as cocatalyst and dimethoxysilane or dicyclopentyl dimethoxysilane as external donor in a 5 liter reactor. (see Examples 1-3 and 3A-10A)
All catalyst according to EP-A-1 717 269 have been prepared under inert conditions in nitrogen atmosphere.
For certain applications, e.g. fibres, it is desirable to have a polymer with even narrower MWD down to 3. A narrow MWD improves the mechanical properties. Further, the narrower the MWD, the faster fibres can be prepared, which results in an increase of the production rate.
Accordingly, although much development work has been done in the field of Ziegler-Natta catalysts, there remains a need for alternative or improved methods of producing ZN catalysts with desirable properties.
It is of particular interest to obtain a catalyst in particulate form which results in good and desired polymer properties and enables the control of MWD, and possibly also further polymer properties, such as content of xylene solubles (XS).
It is therefore an object of the invention to provide a catalyst component, yielding a catalyst with reasonable high activity enabling the production of polyolefins, especially polypropylenes, with narrow MWD values down to 3.0, and preferably allowing also controlling the XS.